While I'm not into competitive weight pull (yet) or sledding, I do have my CO bitch do some pulling just for extra stimuli and exercise. I'd say that with starting out, the biggest thing to remember is not to push the dogs too hard. As Deb and Ren said, baby steps!!! This is not only applicable because of the possible mental problems that might occur, but the physical. Also, keep in mind that it should be FUN for not only you, but your dog. If you've done everything right and started them off slowly, but they still aren't showing signs of not wanting anything to do with it, then it might be best to find another form of work for them to do. Forcing them to do something will only end up undermining your relationship. My CO bitch will pull for me even though she is not too keen about it. So I only do it for short periods of time with her. I do not try to push her too far or long - only as much as I know she will do it. Every time we go out we go slightly further, but I never try to push her more than she's obviously not willing to do. Good luck. :)